chap 3 magnetic sheriff

8/11/2019 Chap 3 Magnetic Sheriff

1/73

Karl Frederick Gauss made

extens

ive stud ies

of

the Earth's mag netic

fi e

ld from about 1830 to 1842

,

and most of bis conclusions are still val

i

d

.

He con-

cludcd from mathematical analysis that the mag netic

eld was entirely due to a source within the

Earth,

rather than outside of it, and he noted a probable

connection to the Earth's rotation because the axis

of the dipole that accounts or mo st of the field is

not far from the Earth's

rotat

ional axis.

The terrestrial

magnetic ficld has beco studicd

almost continuously since Gilbert 's time, but it was

not until

1843

that von Wrede first used variations in

the fteld to locatc dcpos its of magnctic ore . Thc

publication, 1879,

Thal~n

mark ed first use of the magnctic method

.

Until thc late 1940s, magnetic field measurem ents

mostly were made with a

mag ne

tic balance, which

measu red one com ponent of thc carth's eld, usually

the vertical compo nent. This limited measurem cnts

mainly to the land surface

.

Thc ftuxgatc mag netome-

ter was developed during W orld W ar II for detecting

subm arines from an aircraft. After the war, the ux-

gate mag netometcr (and radar

navigation,

another

war developm ent) mad e aeromag netic measu remcnts

possible

.

Proton-precession

magnetometers

,

devcl

oped in the mid-l 950s, are very reliable and their

operation is simple and rapid. They are the mo st

com mo nly used instrum ents today . Optical-pump al -

kali-vapo r ma gnetom eters, which began to be uscd in

1962, are so accurate that instrumentation no longer

limits tbe accuracy of mag netic m easuremcnts. How

ever, proton-prccess ion and optical-pum p m agne-

tometers m casure only tbe mag nitudc, not the direc

tion, of the mag netic field

.

Airborne gradiometer

measu remcnts began in the late 1960s

,

although

ground mcasurcments werc made much earl ier. Thc

gradiom eter often consists of two magnctometers

vertically spaced

1

to 30 m apart. Thc d iffercnce

i

n

read ings not only gives the vertical

gradient

, but

also, to a large cxtcnt, rcmo ves

the

effccts of

tempo-

3.1.2. History of Magnetic Methods

The study the eartb's mag netism

is

the oldest

branch of geophysics. lt has been known for more

than thrcc ccnturies that the Earth bchaves as a large

and somewh at irregular m agnct Sir W illiam Gilbert

(1540-1603) made the first scicntific investigation of

terrestrial mag nctism. He rccorded in d e

that knowledge of the north-sceking property of a

mag netite splintcr (a or leading stone) was

brought to Europe from China by Marco Polo.

Gilbert showcd that the Earth's magnetic

fleld

was

equivalcnt that of a permanent magnct

lying a general north-outh direction near the

rotational axis.

3.1.1. General

Magnetic

and

gravity methods

have

much in

com-

m on, but mag nctics is generally m ore com plcx and

variations in the magnctic ficld are more erratic and

localizcd .

partly due to the difference bctwccn

the dipolar magnetic eld and mooopolar gravity

eld, duc to the variable direction of the

magnetic

field,

wbereas the gravity field is always in

the vertical direction, and partly due to the time-

dependence

of thc magnetic field, whereas th e grav-

ity

field

time-invariant (ignor ing sm all tidal varia

-

tions)

.

W hereas a gravity m ap usually is donnatcd

by regional c.trccts, a magnctic map gcncrally shows

a mu ltitudc of local anomalics . Magnetic me asure-

ments are madc m ore easily and cheaply than m ost

geophysical measurements and correctioas are prac-

tical)y unncccssary. Magnetic

ficld

variations are ot-

ten diagnostic of mineral structures as well as re -

gional

structurcs ,

and the magnetic mcthod is thc

most vcrsatile of gcophysical prospecting technques ,

Howcver, like ali potential

methods ,

magnetic

meth-

ods

lack

uniquencss

of

interpretation.

Chapter


2/73

where

H has the

SI

dimens

in amperes

per

meter

[ - 4w x 10-

3

oersted], and are in meters, 1 is

in amperes, and H, r1 , and

l

ll/ have the directions

indicated in Figure 3.1.

A current flowing in a

circular

loop acts as a

magnetic dipolc

located at

t

he center of the

loop

and

oriented

in

the

direction in

which

a

right-handed

scrcw would advancc

if

tumed

in the direction

of

the

current.

lts

dipole moment is measured in ampere-

meter'' (- 1010 The orbital motions of

electrons around an atomic nucleus constitute circu-

lar currents and cause atoms to have magnetic mo-

AH

(1

x

r

1

/411r2 (3.4)

m

is a

vector

in

the

direction of the

unit vector

r

1

that extends from the ncgative pole toward the

po s

-

tive pole.

A

magnetic field is

a

consequence of the ftow of

an electrical current. As expressed Ampre's law

(also called the Biot-Savart law), a current 1 in a

conductor o length creares, at a point (Fig.

3

.

1),

a magnetizing

eld

J , . f f

given

(3.3)

-

2/pr1

unts); is measured in oersteds (equivalent to

dynes per

unit

pole),

is envisioned as two potes of

strength and

-

separated a distance

21

. Tbc

is defined as

where

4H

is in amperes per meter when 1 is in amperes

. < \ H

(1 d/)

X

1/4w2

F igure 3 . 1

.

Ampere

lew

current through a length of

conductor

creates

a

magnetizing field 4H .ita po

nt

P

:

use a prime to indicate

that

H is in

cgs-em

where F is the force on

2

, in dynes, the poles of

strength

p

and

are

r

centimeters

apart,

.

is the

[a property

of

the mdium: see

Eq. (3.7)), and

r

1 is a unit vector directed from p1

toward P z

-

As in the electrical case (but unlike the

gravity

case, in which the force is always attractive),

the magnetostatic force is attractive for poles of

opposite

sign

and repulsive

Cor

poles of like

sign

. The

sign eonvention is tbat a

pos i t ioe

p < > i e is attracted

toward the Earth's north '1\agnetic

pote;

the

term

is also used.

The H (also callcd

is defined as the force on a unit pole:

(3.1)

3.2.1. Classical versus Electromagnetic

Concepts

Modem and classical magnetic theory ditrer in basic

concepts

Classical magnetic theory is similar to elec-

trical and gravity theory; its basic concept is

that

point magnetic poles are analogous to point electri-

cal charges and point masses

,

with a similar

inverse

-

square law

Cor

the forces between the potes, eharges,

or masses Magnetic units in the centimeter-gram-

second and electromagnetic units (cgs and emu) sys-

tem

are based on this concept, Systme Intemational

(SI)

units are based on the fact that a ma gnetic field

is electrcal in origin.

lts

basic unit is the dipole,

which is created by a circular electrical current,

rather than the ctitious isolated mo nopole of the

egs-emu system. Both emu and SI units are in

currcnt use.

The cgs-emu system begins with the concept of

magnetic force F given

by

Coulomb's law:

PRINCIPLES

ELEMENT

THEORY

field variatons, which are often the limiting fac-

tor on accuracy.

Digital recording and processing of magnetic data

removed much of the tedium involved in reducing

measurements to magnetic

maps

Interpretation

gorithms now make it possible

to

produce computer

-

drawn proles showing possible distributions of

magnetization.

history magnetic surveying is discussed by

Reford (1980) and the state of the art is discussed by

Paterson and

Reeves (1985).

Principies and elementary theorv


3/73

3.2.2. B H Relations: The Hysteresis

The relation bctween B and H can complex in

ferromagnctic matcrials (3.3.5). This is illustrated

by bysteresis (Fig. 3.2) in a

cycJe

of magnetization .

a

demagnctiz.cd

sample is subjected to an

increasing

magnetizing ficld

H

, we obtain the ftrst portion of

the curve in which B increases with

H

until

it

ftattcns

off as wc approach the maximu m valuc that can

have for the samplc W hen H de-

creased, the curve does not retrace thc samc path,

but it does show a positive

value

of B when H

-

O ;

wherc A is a vector arca (A.3.2). lbus

when A

and

B are

parallel, that

is ,

B is the

densi

ty o(

magnctic flux. Thc

SI

unit

for magnetic

ftux

is

the

weber

(

T

-

rrt)

and the

cm

unit

is

the

maxwell

(- 10-1 Wb).

(3.8)

Thcrc is often confusion as to wbether tbe quan

tity involved in magnetic exploration is B or H

Altbough

measure

B . ,

, we

are interested in

the

Earth's fteld H.,.

However, because

B

and

H are

linearly related [Eq. (3

.

7)) and usually I' l

, we

can

(and

do) treat a map of B , . as if it

were

a map

of

H,.

W e

also speak

of

or

4 1 > :

ly - 10-

1

wben H and M

(H

' and M ') are tbc same

drec

-

tion, as is usually the

case.

The SI

unit

for B

is

the

tesla

-

1 newton/ampere-meter

- 1 weber/meter

(Wb/nt).

Tb e elcctrom agnetic unit for B' is the

gauss [

10-4

tesla The permeability of free

spacc has

value

4tr x 10-7 Wb/A-m. In

vacuum

and

in I'l Confusion some-

times results between and because em units

gauss and oersted are numerically equaJ

and

sionally the same

,

althougb conceptuaJly ditrerent;

both H'

and B' are sometimes caJled

thc

"magnctic

eld st:rength." In magnetic prospecting, w e measure

B

to about 10

-4

of

tbc

Earth's main

fteld

(whicb

about

S O

1)

.

Thc unit of magnetic induction

gener

-

ally

used for geopbysical thc nanotesla

(also

B - 0(H M) - l'o(l k)H - l'l'oH

(3

.7a)

B' H' 4trM' - (1 4fl'k')H' l'H' (3.7b)

Susceptibility is tbe fundamental rock parameter in

magnetic prospecting. The magnetic response of

rocks

and minerals is determined by the amounts

and susceptibilities of ma gnetic m aterials in them .

susceptibilities o variou s materials are listed in

Table

3.1,

Section

3 . 3.7

.

The B is tbe total ficld, nelud-

ing

the effect of

magnetization.

can

be

written

(3.6)

M agnetic susceptibility in emu diff ers from that in

SI

units

by factor 4w, that is, called the gamm a,

Figure 3 2

.

Hysteresis

loop

s .

s'

- saturation, r and r'

remanent magnetism, e and e' - coercive force

Or Or' - Residualmaptilin

Oc

Oc' - Coerci~

force

H

(3

.

5)

ments. Molecules

also

bave spin, whlch gives them

magnetic

magnetizable

body

placed extemaJ mag-

netic field becomes magnetized by induction; the

mag netization is due to the reorientation of atoms

and molecules so that their sp

i

ns line up. The ma g-

netization is me asured

by

the

M

(a1so

called or

The lineup of intemal dipoles

produces a field

M,

wbich, within the

body, is

added

lo

the magnetizing field

H. lf M is constant and has

tbe same direction througbout, a body is said to be

The

SI

unit for magnetization

is ampere-meter per

meter

3 [ - ampere per meter

For low magnetic elds

,

M is proportonal to H

and is the direction of

H

. The degree to wbich a

body is magnetized is determined by its

k,

which

defined by

64


4/73

{3.14c)

an a - F,/F, - (1/2)tan9

and the direction with respect to the dipole is

F-

m/r

3 }(1 3 cos2 9)112 (3.14b)

where unit vectors

r1

and

8

are in tbe dircction of

increasing and f J (counterclockwise in Fig. 3 .3).

The resultant m agnitude is

F

QI ( m/r

3

)(2cosfJr

1

sinl&)

(3

.

14a)

where mis the dipole m om ent o f mag nitude

Equations (3.11) and (3.13) give [A .4 and Eq uation

(A.33)]

(3.13)

mcos 2

When r I, Equ ation (3.10) becom es

-p

3/2

( r cos fJ)

r-lcosfJ }

- (3.12a)

2

2rl

cos 9)312

{

I

sin 8

F ,

-

p

3 / 2

(

r

2rl cos

/sin

fJ

}

+

(3.12b)

( r2

1

2r/cos IJ )

lar component

is

these are

Figure 3 . 3 . Calculating tne field ot d megnetic dipote.

-p +p

r.

65

lts

radial component is

F , . -

and

its

angu-

(3

.

11)

(r)

...

-VA(r)

We

can derive

the

vector F

by taking the gradient o

(Eq.

(A.17)]:

-

1/2 (3.10)

2

2/rcos 8)

1

/

2

(r

2

2

- 2/rcosfJ)

p

- . . . )

1 '2

However, since a magnetic pole cannot exist,

we

consider a magnetic dipole to get a realistic entity.

Referring to Figure 3.3, we calculatc

atan

externa)

point

:

(3

.9)

(

r) - -

J ' F(

r)

dr p

-OQ

3.2.3. Magnetostatic Potential for a

Dipole Field

Conceptually the mag netic scalar potentiaJ at the

point is the work done on a unit positive pole in

bringing it from infinity any path against a m ag-

nctic

field F(r)

[compare Eq. (2.4)]. (Henccforth in

cbapter F,

F

indicate mag nctic field rather than

force and we assumc J J - l.) When

F(r)

is

dueto

a

positive pole at a distance from P,

this is called

magnetism.

When

H

is

reversed, B

finally

becom es zero at sorne nega-

t i

v

e value of knowo as tbe The

other of tbe hysteresis loop is

obtained

mak ing still mo re negative until reverse saturation

is reached

and

then retuming

H

to the original

po s

tve

saturation

valu

.

The area

inside the curve

represents the eoergy loss per cycle per unit volume

as a result of hysteresis (see Kip, 1962

, pp

23 5

-

7)

.

Residual

eff ects

in magnetic materials

will

be

dis-

cussed

in more detail in Section 3

.

3

.

6.

In sorne

magnctic

materials ,

B may be quite large as a result

of previous mag netization having no relation to the

present

value

of

H

Principies elementary theory


5/73

where the drectons of F, and F(r0) are not ne cessar-

ily

the same. F(

r

0

)

is much smaller than

F,

or

if

the

body has no residual magnetsm, F and F,

will

be in

approximately the same

direction

. Whcre F(r0) is

an

appreciable fraction (say, 25% or more) of

F,

and

F - F,

F(r

0)

The magnetic

fteld

in Equation (3.20) exists in the

presence of the Earths field

F,.

that

is.

the total

fteld

F is given by

(3.20)

(

d v

)

l r o

-

rl

(Eq.

(A.18)) and

)

V - M- - M (-

(3.19)

Mis a

constant vector

with direction a - ti

+

mj nk. then

the o peration

F(r0

) -

v f .

M(r)

v (

l

(3.18)

r o

-

r

resultant mag netic eld can obtained by

employing Equation (3 .

11)

with Equation (3.17) .

gives

(3.17)

v (

r o

-

r l

the body (Fig,

3.4) is

potential for the wholc

body

at

a

point outsidc

M(r)cos9/r2

- -M(r)

V(l/r) (3.16)

3

.2.4

. The General Magnetic Anomaly

A

volume of magnetic material can be considered as

an assortment of magnetic dipoles that results from

the magnetic moments of individual atoms and

dipoles. Whether they initially are

aligned

so that a

exhib

ts residual magnetism depends on its

previous magnctic history. They will , howcver,

aligned by

induction

in

the

presence of a magnetiz

ing

field . In any case, we may regard the body as a

continuous distribution of dipoles

resulting in a vec-

tor dipolc mo mcnt per unit volume, M , of magnitude

M. The scalar potential at P [see Fig. 3.3 and Eq.

(3.13))

some distance away from a dipole (r

is

(3.15c)

}

m/r3

m/r

3

r l, these simplify to

F ,

-o

(3.15a)

m/(

r2

1 2)

312

6 -

'1/2

(3.15b)

F,-0

Two

special cases, 9 - O

and . , ,/2 in Equation

(3.12),

are called

the

(end-on) and

(side-on)

positions

. From Equations (3.12) they are

given

Figure

3

. 4

.

General magnelic ;moma/y

.

z


6/73

3.3.1 .

Nature the

Geomagnetic Field

As far as exploration gcopbyics concerned, thc

geomagnetic field of the Earth composed of three

parts

1. The main field, wbich vares relatively slowly and

is of intcmal origin.

2.

A small field (compared to the main eld), which

vares rather

rapidly

and originates outside the

Earth.

3 . Spatial variations of the main eld, which are

usually smaller than tbe main eld, are nearly

constant in time and place, and are caused by

local magnetic anomalies in near-surtace crust

of the

Earth

. These are thc targets magnetic

prospecting.

3.3. MAGNETISM

Of

THE EARTH

These relations are used to makc pseudogravity maps

from magnctic

data

M/yp)(

1 (3.28)

In particular,

if

M is vertical, the vertical component

of is

(3.27b)

p

- ( M/yp) U t l / J

where dVda. Por a component of

F

in the

direction

this becomes

{3 .

27a)

F -A -M/yp)Vg,.

... (M/yp)V(VV

ut)

::s (

V

w e apply this result to an extended body, w e

must sum contributions for each element of

v o lu me

.

Provided that M and p do not change throughout

the body, the potentials and will be those for

the extended

body Therefore

, Equations

(3.24)

to

(3.26)

are vaJid for an extended

body

with constant

density and uniform magnetization.

In terms of elds,

(3.26)

Thus,

r Y 1 a.~ e

;

J

Ote..

n

u = t

t

?

m

C:...l~ t'i

f?.b( . . .

H

OYt1 .

(

. 67

nent of g in the direction is

~ ""1

-dU

/

a

-vU

1 ...

-ypV(l/r)

1

(325)

From Equations (2.3a).

(2 .

5), and (A.18), the compo-

A

-M V{l/r) -

-M

V(l/r)

Ui (3.24)

we

have an infinitesimal unit volume with mag-

netic moment

M and density

p,

then at a

distant point we have, f rom Equation (3 .16),

3.2.S. Poisson's Relation

{3 .23)

In

a nonmagnetic medium, M

O

and

(3.22)

2A - 4trV. M(r)

is the net positive pote strength per unit voJume at

a point.

W e

recall that a field F produces a partial

reorientation along thc field direction of the prev

ously

randomly oriented elementary

d

i

poles

.

causes, in efl'ect, a separation of positive and nega-

tive

poles. Por example, the component of F

separates pote strengths and

-

by a distance r

along tbe x axis and causes a net positive pote

strength ( M. d z to en ter the rear

face in Figure A.2a. Because the pole strength

leaving

through

the

opposite face is {

M.

d z , the net positive pole strength

per unit volume ( created at a pont by the field F

is M. Thus,

The magnetic interpretation problem is elearly more

complex than the gravity problem because of thc

dipolar field (compare

2.2 .

3)

.

The

magnetic potential A,

like

the

gravitational

potential V, satisfies Laplace's and Poisson 's equa-

tions

. Following the

method

used to derive Equa-

tions

(2.12)

and (2.13), w e get

2

a 2

~

- k F--

D O 2

viro

- r] ,

a/

2 Viro - rl

(321b)

where 1 is a unit vector in the d

rection o( F,

( 3 . 3 .2a). the magnetization is mainly induced by

F,,

then

has a d fferent direction, the component of F(r

0)

in

the direction of F,, F becomes (3 . 20)}

a A a

2

d o

Fo

- VA . . . . -

a

M a a a f r

(32la)


7/73

(b)

Or

i

gin of

the main field. Spherical harmonic

analysis of the observcd mag netic field show s that

ovcr is duc to sourccs inside the Earth. The

present theory

is

that the

main field is

caused

convection currents of conducting material circuJat

ing i

n

the

liquid outer

corc

(which extends

from

depths of

2,800

to 5,000

km)

. The Earth's core is

assum ed to be a mixture of iron and nickeJ, both

good elcctrical conductors. The mag netic so urce is

thought to a self-cxcited dynam o in which bighly

conductive ftuid mo ves in a com plex manner causcd

by

convection

. Paleom agnetic data show that the

magnetic field has always bcen

roughly along the

Ear th's spin axis, imply ing that the conv ective m o-

tion coupled to the Earth's spin. Rccent explo-

ration of the magnetic

fte1ds

of other planets and

their satellites provide fascinating com parisons

the Earth's field

.

or positive

pole

; the end that

d i

ps downward in

southcrn

lati

tudes i

s

the south -seek ing or negative

pole

Maps

showing lines of equaJ declinat

i

on, inclina-

t

on, horizontal intensity, and so on, are called

(Fig

3

. 6)

.

and

show

,

respectively, lines

of equal

declination inclination and equal values of F,,

H,, or z,

.

Note that the incl i

na

tion is large (that is,

Z, H.) for mo st of the Earth's land m asses, and

hence corrections do not have to be made

C o r

lat-

t

ude variations of ~ or

Z,

(

4

nT/m ) exeept tor

surveys covering extensive arcas

.

overall mag-

netic eld does not reftect variations in surface geol-

ogy,

such as rnountain ranges, mid-ocean ridges or

earthquakc belts, so the source of the main field les

deep witbin the

Earth.

The geomagnetic eld resem-

b

l

es that of a dipole w hose no

r

th and south magnctic

potes are located approximately at 75N ,l01 W and

69S, 145E.

The dipole is

displaced about

300 km

from the Earth's center toward Indonesia and

is

inclined sorne 11.S to the Earth's axis

.

However, the

geomagnetic

field

is

mo re com plicatcd than thc field

of a simple d ipole. The points where a d ip needJe is

vertical, the are at 75N, 101 W and

67S, 143E.

lbe mag nitudes of at the north and south

magnetic poles are 60 and

70

rcspcctively . The

minimum valuc , - 25 occurs in southcrn Brazil

-South

Atlantic

.

In a

few

locations,

F ,

is

larger

than 300 l'T because of near-surf ace m agnetic fea-

tures

The line of zcro inclination (

where O ) is never more than 15 from the

Earth's cquator.

Thc

largcst deviations are Soutb

Am erica and the eastem Pacific

.

In Africa and

it is sligbtly north

of

the equator.

Magnetic methods

stated earlier, the end of the needle that dips

dow nward northern latitudes is the north-seckiog

F, .f.li .f.( co s

D

cos

+sin

/J

+sin

H,sinD

tan/ Z,/H,

X, -

H,cos

D

D

-

Y,/X,

(3

. 29)

F.2

_ z2 _ xi y : 2 z2

' ' '

'

H,

cos

1

Z,

sin 1

3.3.2.

The Main

Field

(a) T h e Earth's magnetic field.

an unmagnctizcd

steel needle could be hung

at

its eenter of

gravity,

so

lhat it is free to orient itself in any dircction, and if

otber magnetic

fields

are absent, it would assume the

dircction the Eartb's total m agnctic eld, a drec-

lion

that

usually

neither horizontal nor

In-line

the geographic meridian. The magnitude of this field,

F . , tbe or of the needle from the

horizontal,

l,

and the angle it mak es geographic

north ( the D, completely define the

main

magnetic

field.

The (Whitham, 1960) are illus-

trated Figure

3.5 .

The eld can also be describcd

in

terms

of

the

vertical com ponent, Z,, reckoned

positive downward , and tbe horizontal

component

,

H,, which always

positive .

X, and are the

components of

H,,

which are considered positive to

tbe ftorth and east, respectively

,

These elements are

re1ated as follows:

Figure

3. 5 .

emem of tbe Ear th 's

magnet

ic

1

1

1

1

1 1

1 1

1 1

r---r-

1 1 ,'

I

North

6 8


8/73

. :

~ --

-

.

- , .


9/73


10/73

\

1

1

I

l

'

/


11/73

3.3.5. Magnetism of Rocks and Minerals

Magnetic anoma1ies are caused by magnetic mincrals

magnetite

and

pyrrhotite) contained

in

the

rocks. Magnetically important minerals are surpris-

ingly

few number

.

Substances can be divided on the basis of their

behavior when placed an

extemal

field

.

A sub-

stance is its is dominated

atoms with orbital electrons orientcd to

oppose

the

extemal field, that is. if it exhibits negative suscepti

bility

. Oiamagnetism

will

prevail if thc net

magnetic moment of all atoms is zero when H is

zero, a situation characteristic of atoms with com-

pletely filled electron

shells.

The most common da

magnetic earth materials are graphite, marble, quartz,

and salt. When the magnctic moment not zero

when H zero, the susceptibility is positive and the

substance The eft'ects of diamag-

netism and most paramagnctism are

weak

Certain paramagnetic clements, namcly iron,

cobalt, and nickel, have such strong magnctic inter-

action

that

the moments align within rairJy large

regions called This eff'ect called

and it is

- 106

times thc eft'ects of

diamagnctism and

paramagnetism

. Ferromagnetism

dec:reases with increasing temperature and disap-

pears entirely at the Curie

temperature

Apparently

erromagnetic minerals do not exist in nature.

The domains some materials are subdivided

into subdomains

that

align

in

oppositc direc:tions so

that their moments nearly cancel; although they

would otherwise be considered ferromagnetic, the

susccptibility

i

s comparatively

low

Such a substance

is 1be only common example is

bematite.

some matcrials, the magnetic subdomains align

in opposition but thcir net momcnt is not zero, either

bec:ause ooe set of subdomains has a stronger mag-

netic alignment than the other or because there are

more subdomains of one

type

than of the other.

Thcse substanccs are Examples of the

type

are magnetite and titanomagnetite, oxides

of iron and of iron and

titanium

. Pyrrhotite is a

magnetic mineral of the second type. Practically all

magneti

c minerals are ferrimagnetic.

thc Canadian Shield, for example, shows a magnetic

contrast to the Western Plains). Many largc, erratic

variations often makc magnetic maps extremely

complex. The sources of local magnetic anomalies

cannot very deep, because temperatures below

- 40

km

should above the the tem-

perature (

5S0q at

which rocks tose

their mag-

netie properties. Thus, local anomalies must

be

asso-

ciated fcatures the upper

crust.

Masnetic

3.3.4 . Local Magnetic Anomalies

Local changes in the main field result from varia-

tions in the magnetic mineral content of near-surface

rocks. These anomalies occasionalJy are Jargc enough

to double

the main

field.

They

usually

do not persst

over great distances; thus magnetic maps gencrally

do not

exhibir

large-scale regional features (although

3.3

.

3. The Extemal Magnetic Field

Most of the remaining small portian of the geomag-

netc field appears to be associated with electric

currents in the ionzed laycrs of tbe upper atmo-

sphere. Time variations of this

portion

are much

more rapid than for the main "permanent" field.

Some effectsare:

1.

A cycle of U years duration that correlatcs

sunspot activity.

2. Solar

diumal variations a period of 24 b and

a range of

30

nT that vary latitude and

season, and are probably controUed action

of

the solar wind on ionospheric currents.

3. Lunar variations with a h period and an am-

plitude 2 nT that vary cyclically throughout

the month

and seem

to

be

associated

with

a

Moon-onospberc interaction.

4. Magnetic storms that are transient disturbances

amplitudes

up

1,000 nT at most latitudes

and even larger in polar regions, where they are

associated with

aur

ora. Although erratic, they of-

ten occur at 27 day intervals and correlate with

sunspot activity. At the beight of

a

magnetic

storm (which may

last

for

several days), long-range

radio reception

is

affected and magnetic prospect-

ing may be impractical.

These time and space variations of thc Earth's

main field

do

not significantly affect magnetic

prospecting except for the occasional magnetic

storm

Diumal variations can

be

corrected for by use o a

base-station magnetometer. Latitude variations ( 4

nT require corrections for higb-resolution,

higb-atitude, or large-scale surveys.

(e) Secular vsristions of tbe main field. Four hun-

years of contnuous of the Earths eld

has established that it changcs

stowty.

The inclina-

tion has changed sorne 10 (75 to 65) and the

dcclination

about

3S (lOE to 2SW and back to

lO"W) during this period The source of this

wander-

ing is

thought

to

be changcs in

convection

currents

in the core.

The Earth's magnetic field has also reversed drec-

tion a number of t imes . The times of many of the

periodic eld reversals have been ascertained and

provide a

72


12/73

3.3

.8. Magnetic Susceptibility

Measurements

(a)

Measurement

of

k

,

Most measurements of

invoJvc a comparison of the sample with a standard.

The simpJcst laboratory method is to compare the

deection produced on a tangent rnagnetometer by a

3.3.7. Magnetic Susceptibilities of Rocks

and Minerals

Magnetic susccptibility is the significant variable in

magnet ics ,

lt plays the same role as density docs in

gravity interpretation. Although instruments are

available for measuring susceptibility in the eld,

they can only be used on outcrops or on rock sam-

ples, and such measurements do not necessarily givc

the

bulle

suscept

ibility of tbc formation.

From Figure 3 . 2, it is obvious that (hence I'

also) is not constant for a magnetic substance; as H ,

increases, k i

ncreases rapidly at

rst,

reaches a maxi-

mum

, and then decreases to z.ero .

Furthermore

, a).

though

magnetizat

i

on curves have the same general

shape, the value of H for saturation vares grcatly

with thc typc of magnetic mineral. Thus it impor-

tant in making susceptibility determinations to use a

value of H about the same as

tbat of

thc Earth's

field .

S

ince tbe errimagnetic

minerals,

particularly

magnetite

, are the main source of local magnctic

anomalies, th

e

re have bcen numerous attcmpts to

establish a quantitative relation between rock sus-

ceptibility and

Fe_,04

concentration. A rougb linear

dependence

(k

ranging from

10-

to

1

SI unit as

thc

volume percent of Fe3

04

increascs from 0.05$ to

35%) is shown in one report, but the scatter is large,

and results from other arcas differ.

Table 3.1 lists magnetic susceptibilities for a vari-

ety of rocks

.

Although there is grcat variation, even

for a particular rock, and wide overlap between

dfferent

types,

sedimentary rocks have the lowest

average susceptibility and basic igncous rocks have

the highest. In every case, the susceptibility depends

only on the amount of Ierrimagnetic minerals pre-

sent, mainly

magnetite,

sometimes titano-magnetite

or pyrrhotite. The values of chalcopyrite and pyritc

are typical of many sulfide minerals that are basi-

cally

nonmagnctic

. It is possible to locate minerals of

negative

susce

ptibility, although the negative values

are very small, by means of detailed magnetic sur-

veys . It is also worth noting that many iron minerals

are only slightly

magnetic

laboratory methods separate residual from induced

magnetization, something that cannot be done in the

ficld.

1 . (TRM), which

re-

sults wbcn magnctic material is cooled below the

Curie point in the presence of an externa] field

(usually the

Earth'

s eld),

lts

direction depends

on the direction of the field at the time aod place

where the rock cooled Remanence acquired in

this fashion is particularly

stable

This is the main

mechanism for the residual magnetization of ig-

neous rocks

2. (DRM), which occurs dur-

ing the slow scttling of fine-grained particles in

the presence of an external field . Varicd clays

exhb

t

this type of

remanence

(CRM)

,

which

takes place when magnetic grains increase in size

or are changed from one Iorm to another as a

result of chemical action al modrate tempera-

tures, that is, below the Curie point. This process

may be significant in sedimcntary and metamor-

phic rocks.

4 . (IRM), which

is the residual left following the removal of an

externa] field (see Fig. 3.2). Lightning stri.kes pro-

duce IRM over very small

arcas

(VRM)

,

wbich is

produccd by long exposure to an external field ;

the buildup of remanence is a Jogarithmic Iunc-

tion of time. VRM is probably more characteristic

of fne-grained than coarse-grained rocks. This

remanence is quite

stable

,

Studies of the magnetic history of the Earth

indicate that the Earth's field has

varied in magnitude and has reversed ts polarity a

number of times (Strangway,

1970) .

Furthermore, it

appears that the reversals took place rapidly

i

n geo-

logic time, because there is no evidence that the

Earth existed without a magnetic field for

icant period. Model studies of a self-excited dynamo

show such a rapid

tumover

. Many rocks have rema-

nent magnetism that is oriented neither in the drec-

tion of, nor opposite to, the present Earth field, Such

results support the plate tectonics theory, Paleomag-

netism belps age-date rocks and determine past

movements, such as plate rot

at

ions

Paleom

agnetic

3 .

3.6. Remanent Magnetism

In many cases, the magnetizaton of rocks dcpcnds

mainly on the present geomagnctic ficld and the

magnetic mineral content. Residual magnetism

(called NRM) oftcn

contributes to the total magnetization, both in

ampl-

tude and direction.

The effect

is complicated

because

NRM depends on the magnetic history

of

the rock.

Natural remanent magnetization may be due to sev-

eral causes

The principal ones are:

Magnetism of the E arth


13/73


14/73

3.4 .2. Fluxgate Magnetometer

This device was originally developed during World

War

1 1

as a submarine detector

,

Several designs bave

been used for

reco

rding diumal variations in the

Earth's eld, for

airborne

geomagnetics, and as

portable ground magnetometers,

The fluxgate detector consists esscntially of a core

of magnetic material. such as mu-metal, pennal1oy,

or

ferrite

, that has a very high permeability at low

magnetic

fields

.

In the most common design, two

cores are each wound wi h primary and secondary

coils,

the two assemblies

being

as nearly

as

possible

identical and mounted parallel so that the windings

are in opposition. The two primary windings are

connected in series and energized by a

low frequency

(50

1,000 current procluced by a constant

current source. The mximum

current

is sufficient to

magnetize the cores to saturation, in opposite polar-

ty, twice each cycle. The secondary colls, which

consist of many turns of fine

wire

, are connected to a

whose output is proportional to

the difference bctwecn two input signals.

The effeet of saturation in the fluxgate elements

i

s

illustrated in Figure 3.7 .

In

the absence of an exter-

nal magnetic ficld, the saturation of the cores is

Typical sensitivity required in ground magnetic in

struments is between

l

and 10 nT in a total eld

rarely larger than 50,000 nT Recent

airbome

appli-

cations

however, have led to the development of

magnetometers with

sensitivity

of 0001 nT. Sorne

magnetometers measure the absolute

eld

, although

this is nota particular

ad

vantage in magnetic survey-

in g .

The earliest devices used for magnetic exploration

were modifications of the mariner's compass, such as

the Swedish mining

compass,

which measured dip I

and declination D

.

lnstruments (such as

which are

essenti

ally dip needles of high

sens

i

tivity) were developed to measure and

but they are seldom used

now

Only the modero

instruments, the ftuxgate,

proton-precession

, and op-

tical-pump (usually

rub

durn-vapor) magnetometers,

will be discussed. The latter two measure the abso-

tute total eld, and the uxgate instrument a1so

generally measures the total

eld

.

3.4.1.

General

3 .4. FIELO INSTRUMENTS

MAGNETIC MEASUREMENTS

Overton, 1981). They achieve great sens

i

tivity be-

cause of the high magnetic moments and low noise

obtainable

at

superconducting ternperatures

.

(b) Measurement

of

remenent magnetism.

Mea-

surement remanent susceptibilty is considerably

more complicated than that of One method uses

an astatic magnetometer, which consists of two

nets

of equal

moment that are rigidly mounted

paral-

lel to each other in the same horizontal plane with

opposing poles. The magnetic system is suspended

a torsin

ftber

. The specimen is placed in various

orientations below the astat

c system and the angular

deflectionsare measured. This device, in effect, mea-

sures the magnetic field gradent, so tbat extraneous

fields

must eitber be eliminated or made unifonn

over the region of the sample. Usually the

entire

assembly is mounted inside a three-component col

system that cancels tbe Eartb's fteld.

Anothcr instrument for tbc

analysi

s of the resid-

ual

component is the

The

rock

sample is rotated at high speed near a small

pickup coil and its magnetic

moment

generates alter-

nating current (ac) the coil. The phase and inten-

sity

of the coil signal are compared with a reference

signal generated by the rotating system The total

moment of the sample is obtained by rotating it

about diff rent axes

Cryogenic instruments for determining two-

axes remanent magnetism have been developed

(Z immerman and Campbell, 1975; Weinstock and

d, and

d ,

are the deections for the sample and

standard,

respectively.

The samples must be of the

same s

i

ze

.

A similar comparison method employs an induc-

tance bridge (Hague, 1957) having s

e

veral

air-

core

coils of different cross sections to

acc

ommodate sam-

ples of different

sizes

. The sample is inserted into

one of the coils and the bridge balance condition is

compared with

the

bridge balance obtained when a

standard sample is in the coil. The bridge may be

calibrated to give susceptibility directly, in which

case the sample need not have a particular geometry

(although the calibration may not be valid for sam -

ples of highly irregular shape) This type of instru-

ment with a large diameter coil is used in field

measurements on

outcrop

The bridge is balanced

first with the coil remote from the outcrop and then

lying on it. A calibration curve obtained with a

standard relates and the change in

inductance

.

prepared sample (either a drill core or powdered

rock

in a tube) with that of a standard sample of

magnetic material (often FeCl

3

powder in

a

test

tube) when the sample is in the Gauss-A position

[Eq. (3.lSa)]. The susceptibility of the sample is

found from the ratio of deflections:

Field instruments

far

magnetic messurements


15/73

FiBure 3.8. Portable fluxgate megnetometer.

symmetrical and of opposite sign near the peak of

eacb half-cycle that the outpu ts from t h c : two

secondary windings cancel. The presence of an exter-

nal field com ponent parallel to the cores causes

saturation

to occur

earlier for

one

half-cycle than the

other, producing an unbalance. The derence be-

tween output

voltages from the

secondary

windings

is

a

series

of

voltage

pulses

wbich

are fed ioto

the

amplifier, as shown in Figure 3.7d. The pulse hcigbt

is

proportional to amplitude of the biasing eld

of

the Earth. Obviously any component can mea-

by suitable orientation of the cores.

The original problem with tbis type of magnc-

tometer - a lack of sensitivity the core -has been

solved t h c : development and use of materials

having sufficient initial permeability to saturate in

small

fields.

Clearly the hysteresis loop should be as

tbin as possible. Thcrc rcmains a relatively

higb

noise level, caused hysteresis cffects in thc core .

The tluxgate

e1emcnts

should be long and thin to

reduce cddy currcnts. Improvcm cnts introduccd to

increase thc

sgnal-to-noise

ratio

include

the follo w -

ing:

l.

By

deliberately unbalancing the two elements,

voltage spikcs are present w ith or without an

ambicnt ficld. The presence of the Earth's fteld

increases the voltage of one polarity mo re

thc other and this

diff

erenee is amplificd.

2.

Because the odd harmonics are canceled fairly

Fisure Principie of the fluxgate

magnetometer

. Note ttut He

-

F e , etc (From

Whitham, 1960.) (a) Magnetzaton of tbe

c ores

. (b) Flux in the two cores for

F ,

(e) Flux in the cores for F . O

.

(d)

fi

tor F . " O . (e) Output volt.Jge tor

F ,

" O .

H, O

ltll

di

H,,.

O

... Tt

A_

V

8

1 " I ' -

neiir.alion

curves for

lluipte

cores

B,,

B,

H, "" 'O

,, B,

Magnetic methods

Prima a .c . Iie

in the 2 coils

H,- O

7 6


16/73

where the factor

2'f1/'Yp = 2

3.487

0

.002 nT/Hz.

Onl

y the total ficld may be mcasured

(3.30b)

The

constant

is the

the ratio of ts magnetic moment to its spin

angular momentum

The value of is known to an

acc

ura

cy

of

0

.

001

%

. Si

nce pre

cise frequency mea-

surements are

relatvely

easy,

the

magnetic field can

be determined to the same accuracy. The proton,

which is a moving charge

,

induces

, in

a coil sur-

rounding the sample, a voltage that vares at the

precession frequency Thus we can determine the

magnetic field from

(3.30a)

havc a net rnagnetic moment that, coupled with their

s

pin, causes them to precess about an axial magnet c

eld .

The proton-precession magnetorneter depends on

the measurement of the free-precession frequency of

protons (hydrogen nucJei) that have been polarized

in a direction approximately normal to the direction

of

the

Earth's

field . When the polarizing field is

suddenly removed, the

protons

precess about

the

Earth's field like a spinn.ing top; the Earth's field

supplies the precessing force

correspond

ing to

that

of gravity in the case of a top The analogy

i

s

llustrated

in Figure 3

.

9

.

The protons

precess

at an

angular

velocit

y known as the

which is proportional to the mag

n

etic field

F, so

that

3.4.3. ProtonPrecession Magnetometer

This

instrument grew out of tbe

discovery

, around

19 45,

of

nuclear magnetic resonance Sorne nuclei

well in a reasonably matched set of cores, the

even harmonics (generally only the second is

sig-

nificant) are amplified to appear as pos

iti

ve or

negative

signals

, depending on the polarity

o

the

Earth's field.

3. Most of the ambient eld is canceled and varia-

tions in the remainder are detected with an extra

secondary winding

,

4

.

Negative feedback of the

amplifi

e

r

outputs

i

s used

to reduce the effect of the Earth's field.

5

.

By tuning the output of the secondary windings

with a capacitance, the second harmonic is greatly

increased: a

phase

-

sensiti

ve detector, rather than

the

d

ifference amplifier, may be used with this

arrangement.

There are several fundamental sources of error in

the ftuxgate instrument. These include inherent un-

balance in the two cores, thermal and shock noise in

cores,

drift in biasing circuits,

and temperature

sensi-

tivity

(1

nT /C

or

less)

.

Thcse disadvantages are

minor, however, compared to the obvious advan-

tages

-

direct readout, no azimuth orientation,

rather

coarse leveling requirements, light weight (2 to 3 kg),

small sze, and reasonable sensitivity. Another at-

tractive feature is that any component of the mag-

netic field may be measured.

No

elaborate tripod is

required and readings may be made very quickJy,

generally in about

15

s. A portable fluxgate instru-

ment is shown in Figure 3.8.

Earth'

1ravity field

Gnv

it.ional

torque on top

---

Gyration

.:

. .

Ma gn e ti

c torque

on proton

Earth's

manetic licld Ma1netc

F momcnt Spin

r Prccess ion momcntum

;th

Field instruments for megneticm e ssurements


17/73

3.4.4. Optically

variety of scientific instruments and tecbniques

ha.s been

developed using

the

energy in transferring

atomic electrons from one energy leve] to anotbcr.

For example, by irradiating a gas with light or

radio-frcqucnc y wav es of the proper frequenc y,

elee

-

trons

may b e raised to a higher cnergy

level.

they

can be

accumulated in

such a state and

thcn sud

denly rctumed

to

a lower level, they releasc som e of

their cnergy in the

process .

This cnergy may be used

for amplification (masers) or to gct an intense light

beam, such as that produeed by a laser.

Thc optically pumpcd magnctomctcr is another

application.

The principie

of

operation ma y be un-

derstood from an examination of Figure

3 .lla,

shows three possible energy levcls,

A

1,

A

2, and

for a hypothetical atom. Under normal conditions of

pressure and temperature, thc atoms occupy ground

state levels

A

1

and

2

The energy

dift"ercnce

be-

twcen

A

1 and

A

2 is vcry small

(,.

10- electron

volts (eV)J

,

represcnting a fine structure

dueto

atomic

electron spins that norm ally are not aligned in tbe

a fixed installation, it posses sorne problcms in small

portable equipment.

The proton-precession mag netom eter's scnsitivity

(

1

nT)

is higb, and it is essentially free from

drift.

The fact that it requires no orientation or lcveling

malees

it attractive

for

marine

and airbomc

opera

tions.

lt

has essentially no me chanical parts, al-

though the electronc com ponents are rclatively com

plex, The main disadvantage is that only the total

field can be measured. a1so cannot record co ntinu-

ously because it requires a sccond o r more between

readings . In

an aircraf

traveling at 300 kmjhr, the

distance interval is abou t 100 m. Proton-precession

magnetometcrs are now thc donnant instrument for

both ground and airbome applications.

The essential com ponents of this magnetometer

include

a

source of protons,

a

polarizing rnagnetic

fteld considerab)y stronger than that of the Earth

and

d

rected roughly norm al to it (tbe direction of

this fieJd can

be

off by 45),

a

pickup coil coupled

tightly

to the source, an amplificr to boost the minute

voltagc inducc d in the pickup coil, and a freque ney-

measurng device

.

The latter operates in the audio

range because, from Equation (3 . 30b), " - 2130

for 50,000 nT. mu st also be capable of indi-

cating frequency diff erences of about 0.4 for an

instrument scnsitivity

10

The protn source is usually a small bottle of

water nuclear mom ent oxygen is

sorne organic

fluid

rich in hydrogen, such as alcohol.

The polarizing

field

of

5

to

10 mT is

obtained

passing direct current through a solenoid

wound around the bottle, w hich is oriented roughly

east-west

C o r the measurem ent. W hen tbe solenoid

current is abruptly cut off, the proton precession

about the Earth's eld is detccted a second coitas

a transient voltage building up and decaying over an

interval of

- 3

s, modulated by the precession fre-

quency

. In sorne models the same coil is used for

both polarization and detcction. The modu lation sig-

nal is amplified to a suitable level and the frequcncy

measurcd. A schematic diagram

is

shown in Figure

3.10.

The measurcment of frequency m ay be carried

out

by

actually counting preccssion cyclcs in an

exact time nterval, or by comparing thcm with a

very

stable frequency

gencrator .

In ene ground

model,

the precession

signa) is

mixed with a signal

from a local oscillator of high precision to produce

low-frequenc y beats (

100 that

driv e a vibr at-

ing reed frequency meter. Regardless o f the method

used, thc frequency must be measured to an accu-

racy

0 001 %

realize the capabilities of tbe

method .

Althougb this is not particuJarly difficult in

Figure

3 . 10.

Proton-prece

ss

ion magnetometer

(From

Sheriff

, 1984.)

Co.nter to ''" 11tt

crctu

~ ... '"''

1111110~

Magnetic

T . , . ,

l

t

ctnlrtl

' r . , , , 1 ,

t o . . - - - _ . , . . _ _ ,

11111u11co11,

tltrOI 1111,11

78


18/73

wh

ere y, i

s

the

For

Rb, the vale of y,/2.,, is approxirnately 4.67

Hz

T

whereas

the corresponding frequency for

~

50,000 nT is

2

33 kHz. Because )', for the

electr

on

is

known to a precision of about

1

part in 10

7

and

because of the relatively high frequencies

involved,

it

(

3

.

31)

the energy levels

1

and

2

(actually the sublevels

are more complicated

than thi

s ,

but

the simplifica-

tion illustrates the pumping ac

tion adequately),

and

there is a difference of

o

ne quantum of angular

momentum between the parallel and antiparallel

states The irradiating beam is circularly polarized so

that

the photons in the light bearn have

a

single

spin

axi

s . Atom

s

in suble

ve }

then can be pumped to B.

gaining one quantum by absorption

,

whereas those

in 2 already have the same momentum as and

cannot make the transition.

F

igure 3.12 is a schematic

d

iagrarn of the rubid-

ium-vapor magnetometer. Light from the lamp is

circularly polarized to illuminate the Rb vapor cell

after which it is refocused on a photocell, The axis of

this bearn is

inclined

approxima

tely 45

to the Earth's

field, which

c

auses the

electr

ons to precess

about

the

axis of the eld

a

t the Larmor requency. At one

point in the precession c

ycle

the atoms

w

ill be most

nearly parallel to the l i

ght-b

eam d

i

rection and one-

half cycle later they will be more

antiparallel.

In the

rst

po

sition. more light is

tra

nsmitted through the

cell

than

in the second.

Thus the

precession Ire-

quency produces a

v

ariable light

intens

i

t

y

that

ick

-

ers at the Larmor frequenc

y

. If the photocell signa) is

amplified and fed back to

a

coil w

o

und on the cell,

the

coil-mplifier

sy

stem becomes

an

oscilJator

whose frequency

is given

by

same direction. Even tbermal

energie

s (

=

10 -

2

eV)

are much larger than this, so that the

a

toms are as

Jikely to be in level

1

as in

2

.

Leve B represents a much higher energy and the

transitions from 1

or

2 to correspond to in-

frared or visible spectral

lines

. we irradiate a

sample with a bcam from which spectral line A

2

B

has been removed, atoms in le ve) A can

absorb

energy and rise to B . but atoms in

A

wiU not be

excited, Wben

the excited

atoms

fall

back to

ground

state, tbey may return to either level, but if they Iall

to they wiJJ be removed by

photon

excitation to

8 again. The result is an accumulation

of

atoms in

level

A 2

The technique of overpopulating one energy leve]

in

this Iashon

is known

as

As thc

atoms are moved from leve) A

1

to

A

2

by this selec-

tive

process,

1ess energy will be absorbed

and

the

sample

bec

ornes increasingly trans

parent

to the

irra

-

diating

beam

When ali atoms are in

the

A 2 state,

a

photosenstive detector will register a maximurn cur-

rent, as shown in

F

i

gure

3.llb

. now we apply an

signal, having energy correspond

i

ng to the

tran

-

sition between

A

1

and

A

2

,

the pumping effect is

nu11ified

and the transparency drops t

o

a mnimum

again. The

proper

freque

nc

y for this signal is given

by si - E/h. where E is the energy difference be-

tween A

1

and A

2

and

is

Plan

c

k

's

constant 1 6

. 62

34

joule-

se

conds] .

To malee

this

dev

i

ce into a

magnetometer, it

is

necessary to select atoms that have magnetic energy

sublevels

that are

suitably s

paced to give a measure

of the wealc magnetic

field

of the Earth.

Elements

that have been used Ior this purpose include ce si

um

rubidium

,

sodium. and

heliurn .

The first three each

have a single electrn in the outer shell whose spi

n

lies either parallel or antip

a

rallel to an external

magnetic

eld

These two orient

a

tions correspond to

Figure J 1 1 .

Opt

i

c

el pumping .

(

a) fn

er

gy le v e trens ttion s .

(

b) ol

p

umping 0 1

lisht

(b)

79

ar

-B

I t / J

M in .

Random Microammeter cumut

distribution 5

~

t . - - ~ 1 r - - j - .

~

/~f.{~4 : : = ~ -_ :

:

~::Y'LJ

= : r

FilW' A Pbococell

2 spoctral cell Mu .

'

; ~

:

c ; . = : : - ~ : _

:

~ _ ( _ , _ ~ - _ ; ; : 1 _ ~ _ 1 . . . J i f ~ i l k J t - o - [ i

M in

.

1 1 "I

j1-

_; .Ai=1t-i----=--

-4-:~

1 1

Pumpinanulli1ied_ '

1 1 " " _ ' . . . . : , ~ . : _ = - - : _ - - l f : = : i :

.r

-v-

by RF

s i

pal;-- ~

1 1

A 1

A1

sisnaJ


19/73

Magnetic exploration is carried out on land, at sea,

and in thc

air.

For arcas of appreciable

extent,

surveys usually are done with the airbome magne-

torneter.

In oil exploraton, airbome magnetics (along with

surface is done as a preliminary to seismic

work to establish approximate depth, topography,

and character of the basement rocks. Since the sus-

ceptibilities of sedimentary rocks are relatively small,

the main response is dueto igneous rocks below (and

sometimes within) the sediments.

Within thc last few years it has become possible

to extraer from aeromagnetic

data

wcak anomalies

originating in sedimentary rocks

,

such as result from

tbe faulting of sandstones

This results from (a) the

improved sensitivity of magnetometers, (b) more

pre

cise determination of location Doppler radar

(B.S), (e) corrections for diumal and other temporal

3.5 .1. General

3.5. FIELD

OPERATIONS

(332b)lH 9 .0N ll/a

whcre is in microamperes, in nanoteslas, and a

in mcters. Bccause varies

directl

y with thc cur-

rcnt, this can be written

(3.32a)

a

9 .

0Nl/a

method ernploys a large enough to

surround the instrument. This is a

pair

of identical

coils of N turns and radii coaxially spaced a

d

i

stance apart

equal

to

the rad

i

us

.

The

result

i

ng

magnetic eld, for

a

current I flowing through the

coils connected in series-aiding, is

dire

cted along the

axis and is uniform within about 6~ overa cylinder

of diameter and lcngth concentrlc the

coils. This tield is gven by

3

.

4.7

.

Calibration of Magnetometers

Magnetometers may be calibrated by placing them

a suitably oriented variable magnetic field of

known valuc

The most dependable calibration

3.4.6.

lnstrument Recording

Originally the magnetometer output in airbome in-

stallations was displayed by pen recorder. To achieve

both bigb sensitivity and wide rangc, the graph would

be "paged back" (the relerence value changed) Ire-

quently to prevent the from running off the

paper. recording is done digitally, but gener-

ally an analog display is also made during a survey.

Some portable instruments for ground work also

digitally record magnetometer readings, station coor-

dnates, diumal corrections, geological and terrain

data.

3.4.5. Gradiometers

The sensitivity of the optically pumped magnetome-

ter is considerably greater than normaJly required in

prospecting. Since

1965,

opticaJly pumped

rubidium-

and eesum -vapor magnetometers bave ncreas-

ing)y empJoyed in airbome

gradiometers.

Two detec-

tors, vertically separated by about

35 m ,

measure

dF

/dz, the

total-eld

vertical gradient. The sens

tv-

ity is rcduced by pitcb and yaw of the two birds.

Major improvements by the

Geological

Survey of

Canada involve reducing the vertical separation to

1

to

2

m and using a more

rigid

conncction bctween

lhe sensors. Gradient measurements are also made in

ground

surveys

. The two sensors on a staff in the

Scintrex MP-3 proton-magnetometer system, for ex

-

ample, measure the gradient to 0.1 nT/m Gra-

diometer surveys are discussed further in Section

is not difficult to measure magnetic field variations

as small as 0.01 nT

w

ith a magnetometer of this type

F igure 3 . 12

.

Rubidium-vspor magnetometer (schemetic)

1lecordcr ---

Bi

as Frequcncy __ _.

1.---

Magnetic


20/73

(e)

Effect

of

variation

s in

fligh

t

path

Altitude

differences between ftight lines

ma

y cause herring

-

bone pattems in the magnetic

data .

Bhattac

haryy

a

(1970) studied errors arising from tligh

dev

iations

td)

Flight pattern.

Aerornagnetic surveys almost

always

consis

t

of parallel lines (Fi

g

. 3. 1

Jc)

spaced

anywhere from 100 m to severa) kilometers

apa

rt.

The heading generally is normal to the main geologic

trend in the area

and altitude

usuaJly is maintained

al

fixed elevations, the height being continuously

recorded by radio or barometric altimeters, lt is

custornary to record changes in the Earth's eld with

time

(dueto diumal or

more sudden variations) with

a recording magnctometer

on

the ground A further

check generally is obtain

e

d by fl y ing severa) cross

lines, wbich verify readings at line

intersecti

ons,

A which approximates constant

clearance over rough topography, is generally own

with a helicopter. lt is often assumed that drape

surveys minirnize magnetic terrain eff ects,

but

Grauch

and

Campbell (1984) dispute this. Using a uniformly

magnetized model of a

mountain-val

ley

s

ystern, four

profiles ( one leve), the others at diff erent ground

clearance) ali showed terrain effects. However,

Grauch and Campbell recommend drape surveys

ovcr level-fl

ght

surveys because of greater sensitivity

to smaJI targets, particularly in valle

ys

, The disad-

vantages of draped surveys are higher cost, opera-

tional problems

,

and less

sophist

icated

interpretation

techniques.

(e)

Stabilization

. Since proton-precession and

op

ti-

cally magnetometers measure

total

field, the

problem o stable orientation

of

the sensing element

is

minor

.

Although the polarizing field in the

proton-precession i

nstrument must not be

parall

el to

the

total-

eld direction, practically any other orien-

tation will do because the signal amplitude becomes

inadequate

o

nly within a cone of about 5

Stabilization of the uxgate magnetometer

i

s more

difficult

, because the sensing element

mu

st be main-

tained accurately in the F axis. This is accomplished

two additionaJ ftuxgate detectors that are

or

-

ented orthogonally with the first;

that

is, the three

elernents form

a

three-dimensional orthogonal coor-

dinate system. The set is mounted on a small plat-

form that rotares freely in a 1 1 directions. When the

sensing uxgate is accurately aligned along the

total-eld axis, there is zero signal in the other t

wo

A ny tilt away from this axis

produces

a signal in the

c

o

ntrol

elements

that

drive servomotors to restore

the

sys

tem to the proper orientat

ion

.

rnounting Jocation. Figure 3.13b shows an installa-

tion with the magnetometer head

in

the tail.

81

(b} lnstrument mounting. Aside from stabiliza-

tion , there are certain problems in mounting the

sensitive magnetic detector

i

n an airplane, because

the latter has a complicated magnetic eld of its

own. One obvious way

to

eliminate these effects is to

tow the sensing element some distance behind the

ai

.

craft. This was the original mounting arrangement

and is still

used

. The detector is housed in a stream-

lined cylindrical container, known as a con-

nected by a cable 30 to 150 m long. Thus the bird

may be 75 m nearer the ground than the aircraft.

A

photograph of a bird mounting is shown in Figure

3.l 3a.

An altemative scheme is to mount the detector on

a wing tip or slightly behind the tail, The stray

magnetic effects of tbe plane are minimized by

per

-

manent magnets and soft iron or permalloy shielding

strips, by currents in compensating coils, and

by

metallic sheets for electric shielding of the eddy

currents

.

The shielding is a cut-and-ry

proces

s

,

since

the magnetic effects vary with the air

c

raf t and

J5.2. Alrborne Magnetic Surveys

(a ) General In Canada and sorne other countries,

govemment agencies have surveyed much of the

country and aeromagnetic maps on a scale of

1

rnile

to the inch are available

at

a nominal sum. Large

areas in ali

parts

of the world have also becn sur-

veyed in the course of oil and mineral exploration .

The sensitivity of airborne magnetometers is gen-

erally greater than those used in ground explora

-

tion

-

about 0.01 nT comparcd w ith 10

to

20 nT.

Because of the initial large cost of the aircraft and

availability of space, it is pra

c

tical to use more

sophisticated equipment than could be handled in

portable

instruments;

their greater scnsitivity is use-

ful in making rneasurements severa hundred meters

above the ground surface, whereas the same sensitv-

ity is usuaJly unnecessary (and rnay even be undesir-

able) in ground

surveys

.

field variations, and

(d)

computer-analysis tech-

niques to remove noise effects,

Airborne reconnaissance for minerals frequently

combines magnetics with airborne In most cases

of ollowup, detailed ground magnetic surveys

are

carried out. The method is usually indirect, that is,

the primary interest is in geological rnapping

rather

than the mineral concentration per se. Frequently

the association of characteristic magnetic anomalies

with base-metal suldes,

gold

, asbestos, and so on,

has bcen uscd as a marker in m ineral exploration ,

There is also, of course, an application or magnctics

i

n the direct search for certain iron and titanium

ores.

Field operstions


21/73

'

1

1

I

I


22/73

(b)

Correcti

o ns

. In precise w ork, either repeat

readings shou

l

d be

made

every few hours at a prev-

ously occupied

station or

a

base-station record

ing

magnetometer should be emplo

y

ed. This providcs

corrections for

diumal and

errat

i

c variations

of

the

magnetic eld , However, such precautions are un-

necessary in most mineral prospecting because

anomalies are large (

>

500

nT)

Since most ground magnetometers have a sensi-

tivi ty of about 1

nT

, stations should not be locatcd

near sizeable objects

containing

iron, such as

railroad tracks, wire fences, drill-hole casings, or

culverts. The instrurnent operator should also not

wear iron articles, such as belt buckles, compasses,

knives

, iron rings, and even steel spectacle

frames

.

Apart

from

d

i

urn

a) eff ects, the reductions

re

quired

fo

r

magneti

c

data

are

i

nsignicant. The vert-

cal gradient vares from approximately 0.03 nT/m at

the poles

to

001

nT

m at the magnetic equator. The

3.5.4. Ground Magnetic Surveys

(a) Genera l

Magnetic surveying on the ground

now almost exclusively uses the

portable

proton-pre-

cession magnetometer . The main application is in

detailed surveys for minerals, but ground magnetics

are also

employed

in the followup of geochemicaJ

reconnaissance in base-metal search Station spacing

is usually 15 to 60

m

occasionally it is as small as 1

m. Most ground surveys now measure the total field,

but vertical-component ftuxgate instrumenta are also

used. Somet

imcs gradiometer measurements

( 3

.

5 .

S)

are made.

3.5 .3. Shipborne Magnetic Surveys

Both the fluxgate and

prot

on-precess

ion magnetome-

ters have been used in marine operations

There are

no major problems in ship

ins

tallat

i

on. The sensing

element is towed sorne distance (150 to 300 m) astern

(to reduce magnetic effects of the vessel) in a water

t

i

ght housing called a fish which

usual

ly

rid

es about

15 below the surface

Stabilization is similar to

that employed in the airborne

bi

rd. U se

o

f a ship

rather than an air

c

raft

provide

s no advantage and

incurs considerable cost increase unless the surve

y

is

carried out in conjunction with

other

surveys

, such

as

gravi

ty

o

r

seism

ic, The rnain application has been

in large-scale oceanographic surveying related to

earth physics petroleum search. Much the

ev idence supporting plate tectoni

c

s has come from

marine

magnetics

ing small areas may be prohibitive The attenuation

of near-surface features, apt to be the survey

ob

jec-

tive , becorne

l

i

m

itations in minera

l

searc

h

(h) Advantages and disad

v

antages of

Airbome surveying

i

s extremely attrac-

tive reconnaissance because low cost per kilo-

meter (see Table 1 .

2)

and

bigh

speed, The speed not

only reduces the cost,

but

also decreases the effects

o r time variations of the magnetic

eld

. Erratic

near-surace Ieatures,

frcquently a

nuisance in ground

work, are

considerably

reduced. The

ftight elevation

may be chosen to favor structures of certain size and

depth, Operational problems associate

d

with irregu-

lar terrain, sometimes a source of difficulty in ground

magnetics, are

minimized

. The data are smoother,

which may malee interpretation easer, Finally, aero-

magnctics can be uscd over water and in regions

inaccessible for ground work.

The disadvantages in

airbome

magnetics

apply

mainly to mineral exploration. The cost for survey-

(g)

ro

magnetic

d a t a

. Magnetic data

are corrected for drift, elevation, and line location

differences at line intersections in a

least-

squares

manner to force

tics

lnstrument drift is generally

not

a

major

problem

, especially with proton and

optically pumped magnetometers whose

me asur e-

ments are absolute values.

The valuc of the main magnetic eld

of

the

Earth

is often subtracted from measurement

values

The

Earth's

eld is

usualJy

taken to

be

that

of the

(IGRF)

model

.

A

stationary

base

magnetometer is

often used to

determine slowly varying

diurna)

effects. Horizontal

gradiometer arrangements help in eliminating rapid

temporal

variations

; the gradient measurernents do

not invoJve

diumal

effects. Usually no attempt is

made to correct for the large effects of magnetic

storms.

(f) Aircraft

The s

i

mplest method of locat-

ing the aircraft at ali times, with respect to ground

location, is for the pilot to control the flight path by

using aerial photographs, while a camera takes pho-

tos on strip to determine locations late

r

The

photos and magnetic data are simultaneously tagged

at intervals. Over featureless terrain , radio naviga-

tion (see B.6) gives aircraft position with respect

t

o

two or more ground stations, or Doppler

radar

(B.5)

determines the precise flight path. Doppler

radar

increasing)y is employed where high accuracy is re-

quired.

over an idealized dike (prism)

target .

Altitude and

heading changes produced eld measurement changes

that wou)d alter interpretations based on anomaly

shape measurements, such as those of

slope

Such

deviations are especially significant with high-resolu-

tion

data

.

Field operetions


23/73

3.6.1. General

Because ground surveys (until about 1968) measured

the

vertical

-field

componcnt

, whereas airborne sur-

veys measurcd the total eld, both vertical-compo-

ncnt and total-eld responses will be developed

Depth detenninations are most important and lateral

extent Jess so, wbereas dip estimates are the lcast

important and quite difficult

In this regard, aero-

magnetic and electromagnetic interpretation are sim-

ilar. In petroleum exploration the

depth

to bascment

is the prime concern, whereas in mineral exploration

more detail is

desirable

.

The potentialities of

high

rcsolution and vertical-gradient acromagnetics are

only now being exploited to a 1imited extent.

\sin gravity and

clec~ctics

an2m1llies&&..

often matched w ith models

The magnetic_problem is

ioiCdi'~

[

~~ciH~~J f i ~ -91~~

-

aiaiac~C-

~~~

~

~ -~~}d_~d the possi~~ o~~

manencc

Very

simple geometncal mooels are usual]y cmpJoyed:

isolated pote, dipole, lines o poles and dipoles, ttn

plate, dike (prism), and vertical contact. Becausc the

shape of magnetic anomalies relates to the magnetic

fieJd,

directions in the following sections are with

respect to magnctic north (the direction), magnetic

east, and so

forth,

the z axis is positive

downward

,

and we assume that locations are in the northem

hemisphcre

Wc use 1 for thc field inclination, (

3

.

6.

MAGNETIC

EFFECTS

OF

SIMPLE

SHAPES

cent. For the vertical contact, balf the separation

between maximum and minimum

vales

equals the

depth. Gradiometer measurcments

are

valuable in

field continuation calculations

(3. 7 . 5) .

Ground

gradi

ometer measurements (Hood and

McClure, 1965) have recently been carried out for

gold

deposits in castem Canada in an arca

where

tbe

overburden is only a ew meters thick, The host

quartz was located because of its slightly negative

susceptibility using a vertical separation of 2 m and

a station spacing of

1

m. Gradiometer survcys

bave also been used in the search for archeologcal

stes

and artifacts, mapping buried stone structures,

Jorges, kilos, and so forth (Clark, 1986; Wynn, 1986).

Vertical gradient acromagnetic surveys (Hood,

1965) are often carried out at 150 to 300 m aluuide.

Detailed coverage with 100 to 200 m line spacing is

occasionally obtained at 30 m ground

clearance

.

Two magnetometers horizontally displaced from

cach other are also

used

, especially with marine

measurements where they may be separated by 100

to 200

m

This arrangement permits the elimination

of rapid temporal variations so that small spatial

anomalies can be interpreted with higher confidence.

Magnetic

where

Jj

and

f 2

are readings

al

the higher and lowcr

elevat

i

ons, and is the separaton

distance

Discrimination between neighboring anomalics is

enhanced in the gradient

measurements.

For exam-

ple

,

the anomalies for two isolated poles at depth h

separated by a horizontal distance h yield separate

peaks on a a F I a profile but they have to be

separated by 1.4 h to yield separate anomalies on an

profile

The effect of diumal variations is also

minimized, which is especially beneficial in high

magnetic latitudes. For most o the simple sbapes

discussed in Scction 3.6 (especially for thc isolated

pote, finite-ength dipole, and vertical contact o

great depth extent), better depth estimates can be

made from the first vertical-derivative proles than

from either the Z

or

F proles, For features of the

first two types, the width of the profile at

(az;az)mu./2 equals the depth within a few per-

(F2 - F)/4z

3.5 . 5. Gradiometer Surveys

The gradient of F is usually calculated from the

magnetic contour map with tbe aid of templates.

Thcre is, however, considerable merit in measuring

the vertical gradient directly in the

eld,

It is merely

necessary to record two readings, one abovc the

other

. With instrument scnsitivity of

1

nT, an eleva-

tion difference of

o

1 m suffices

.

Then the vertical

gradient is given by

Z(

X, y

. O ) - Z(

X

,

y.

h)

- h( a zaz)z_, ,

(3

.

33)

latitude variation is rarely 6 nTjkm. Thus eleva-

tion

and latitude

corrections

are generally unneces-

sary

The inftuencc of topography on ground magnet-

les,

on the other hand, can very important. Th.is is

apparent when taking measurements in stream

gorges, for example, where the rock

wa11s

above the

station frequeotly produce abnormal magnetic lows

.

Terrain anomalies as large as 700 nT occur at steep

(45) slopes of only 10 m extent in formations con-

taining 2% magnetite (k - 0025 SI unit) (Gupta and

Fitzpatrick, 1971). In such cases, a tcrrain correction

is

required, but it cannot applied merely as a

unction of topography alone because there are situ-

ations (Ior example, scdimentary formations of vcry

Jow susceptibility) in which no terrain distortion is

observed

A terrain smoothing correction may b

chap 3 magnetic sheriff

Documents